1. Field of the Invention
The present invention relates to a cathode-ray tube display apparatus using a cathode-ray tube such as a projection TV system and a screen-noise eliminating apparatus.
2. Description of the Related Art
Recently, high-definition broadcasting services with more than 1000 scanning lines have become available for viewers along with the conventional NTSC broadcasting services with 525 scanning lines, and television systems capable of receiving both the services are being used. Among such television systems, in those using cathode-ray tubes (hereinafter referred to as “CRT”), the size of the bright spot (hereinafter referred to as “beam spot”) on a phosphor surface generated by an electron beam of the CRT is determined such that adequate display of those high-definition signals with a large number of scanning lines can be obtained, thus, when displaying an NTSC signal having less scanning lines, the use of the same beam spot size would result in gaps on the screen, over which the electron beam does not scan. In order to prevent these gaps, the beam spot size is made larger or the beam spot is wobbled (hereinafter referred to as “wobbling”) in the vertical direction across the trace along the scanning lines when displaying a signal having a smaller number of scanning lines.
FIG. 9 is a schematic block diagram indicating a configuration of a prior art projection TV system, which is one of CRT display devices. Illustrated in the diagram are, a TV signal 1, a signal processing circuit 2 for correcting the contrast and brightness of the TV signal 1 and converting it into primary-color signals, a driving circuit 3 for amplifying the signal level for each of the primary-color signals and driving the CRT cathode, a CRT section 4 corresponding to green, blue and red colors, which comprises a CRT 4A, a horizontal deflection coil 4B, a vertical deflection coil 4C and a second vertical deflection coil 4D, a sync separation circuit 5 for separating a synchronization signal from the TV signal 1, a sync generation circuit 6, a horizontal sync signal 7, a vertical sync signal 8, a horizontal deflection driving circuit 9 for driving the horizontal deflection coil 4B according to the horizontal sync signal 7, a vertical deflection driving circuit 10 for driving the vertical deflection coil 4C according to the vertical sync signal 8, a control circuit 11 which identifies the number of scanning lines from the inputted horizontal sync signal 7 and vertical sync signal 8 and outputs a wobbling amplitude control signal, and a second vertical deflection driving circuit 12 for providing high-speed, fine deflection of scanning lines in the vertical direction according to the wobbling amplitude control signal.
The TV signal 1 is inputted into the signal processing circuit 2 which in turn corrects the contrast and brightness of the signal, converts it into primary color signals to be outputted to the driving circuit 3. The driving circuit 3 amplifies the signal level of each primary color signal and outputs it to the CRT section 4. The TV signal 1 is also inputted into the sync separation circuit 5 which separates the sync signal from the TV signal, and the separated sync signal is inputted into the sync generation circuit 6 which outputs the horizontal sync signal 7 and the vertical sync signal 8 generated from the inputted sync signal. The horizontal sync signal 7 is inputted into the horizontal deflection driving circuit 9 which drives the horizontal deflection coil 4B based on the inputted horizontal sync signal 7. The vertical sync signal 8 is inputted into the vertical deflection driving circuit 10 which drives the vertical deflection coil 4C based on the inputted vertical sync signal 8. The control circuit 11 receives inputs of the horizontal sync signal 7 and vertical sync signal 8 and identifies the number of scanning lines, and when the number is found to be smaller than the maximum displayable number of scanning lines, it outputs the wobbling amplitude control signal to the second vertical deflection driving circuit 12 which, in turn, controls the vertical deflection coil 4D to cause the high-speed, fine wobbling of the beam spot in the vertical direction according to the wobbling amplitude control signal.
FIG. 10 shows a set of diagrams illustrating a known scheme for scanning a TV screen and the effect of wobbling in which FIG. 10(A) indicates a screen and scanning lines of a typical cathode-ray tube display apparatus. In the diagram, a display screen 13 and scanning traces 14 of scanning lines are illustrated. FIG. 10(B) is an expanded view of the scanning lines, in which numeral 15 indicates the beam spot on scanning lines, numeral 16 indicates the passing range of the beam spot 15, numeral 17 indicates those areas of the screen that are not scanned by the beam spot 15, and numeral 18 represents the traces of the scanning lines when wobbling is performed. FIG. 10(C) is a diagram showing the beam spot positions and the light intensity. In this diagram, the curves 19 represent the light intensity distributions of the beam spot 15, and the curves 20 represent the light intensity distribution with its extent increased in a vertical location of the screen as a result of the wobbling.
Where a TV signal 1 with a maximum number of scanning lines is inputted into the cathode-ray tube display apparatus, the second vertical deflection driving circuit 12 does not operate, so that the scanning lines would be scanned along the linear traces 14. On the other hand, where a TV signal 1 with less scanning lines is inputted into the cathode-ray tube display apparatus, the wobbling takes place, causing the beam spot 15 to be scanned along the traces 18. While unscanned regions 17 would occur on the screen if the wobbling is not implemented, wobbling with an adequate frequency and amplitude will eliminate such regions 17. The light intensity distribution of the beam spot 15 is represented by the curves 19, but the high-speed, fine wobbling of the beam spot 15 in the vertical direction would cause the light intensity distribution to temporarily move as shown by the curves 20. Furthermore, with a speed sufficiently fast, this movement of the beam spot 15 would not be perceived, and the light intensity would appear to be even, so that as a result of the wobbling, the beam spot 15 is seen as if it has expanded over the regions 17, eliminating the undesired gaps between the scanning lines 14.
FIG. 11 is a graphical representation plotting the frequency analysis result of the wobbling effect for a frame image displayed on e.g. a conventional projection TV system. In the diagram, the horizontal axis represents the spatial frequency in the vertical direction of the screen, the vertical axis represents the relative intensity of the displayed image spectrum, the point fv1 on the horizontal axis represents the maximum vertical frequency with a cycle of two scanning lines, and the point fv2 represents the harmonic spurious frequency twice the fv1, with a cycle of one scanning line.
When a plane signal is displayed on the display screen, the spatial frequency in the vertical direction of the screen essentially has no alternating current component but only a direct current component as indicated by the point 21. However, when the wobbling is not performed, the light intensity distribution is uneven as indicated by the curve 19 of FIG. 10(C), and this yields a spurious component indicated by the point 22.
Generally, when the screen is viewed from a distance farther than the adequate viewing distance which is said to be about 7 times of the screen height in the case of NTSC and about 3 times of the screen height in the case of high definition, the spurious component at the point 22 is difficult to be observed since it is cut due to the low-pass characteristic of human eyes, thus it unlikely causes the degradation of the image quality. However, when the screen is viewed from a distance closer than the adequate viewing distance, it would show as annoying horizontal-line noise. Especially, when an NTSC signal displayed on a cathode-ray tube display apparatus is viewed at the adequate viewing distance for high-definition display, such horizontal-line noise would stand out as the screen is now viewed from a distance closer than one half of the adequate viewing distance for NTSC signals.
When the wobbling is implemented, in the above case, the light intensity distribution of the beam spot 15 becomes as shown by the curves 20 in FIG. 10(C), so that the unevenness in the light intensity distribution in the vertical direction can be reduced. As a result of this, the spurious 22 reduces to the level of the point 23 or point 24 in accordance with amplitude of wobbling, the horizontal-line noise can be decreased.
Generally, an image signal has a frequency bandwidth of fv1 or smaller in the vertical direction. In FIG. 11, the curves 25 through 27 represent the spectrum intensity of an image signal for each type of frequency components, and it is displayed in the intensity indicated by the curve 25 when the wobbling is not performed, and in the intensity indicated by the curve 26 or 27 when the wobbling is performed. As can be seen from the graph, when the wobbling is performed, the high band of the image signal is reduced as spurious reduces.
FIG. 12 is a graphical representation plotting the spatial resolution of human eye as converted into the spatial frequency on a screen. In the diagram, the horizontal axis represents the TV screen spatial frequency, the vertical axis represents the relative response, the curve 28 represents the response at the adequate viewing distance, the curve 29 represents the response at a location closer than the adequate viewing distance, and the curve 30 represents the response at a location farther than the adequate viewing distance. All of them exhibit the low-pass characteristic, but with the band varied according to the viewing distance.
Accordingly, viewing a screen display implementing no wobbling from a location closer than the adequate viewing distance corresponds to observing the image signal indicated by the curve 25 and the spurious component indicated by the point 22 in FIG. 11 with a response indicated by the curve 29 in FIG. 12. Therefore, while the image signal 25 may be viewed clearly, the spurious component at the point 22 is also viewed strongly, so that the horizontal-line noise would stand out.
Furthermore, viewing a TV screen implementing the wobbling from a location closer than the adequate viewing distance corresponds to observing the image signal indicated by the curve 27 and the spurious component 24 in FIG. 11 with a response indicated by the curve 29 in FIG. 12. In this case, the horizontal-line noise would not be observed since the spurious 24 is at a lower level, however, the image quality is degraded since the high band of the curve 27 image signal is reduced. In addition, the curve 27 may be corrected to the level of the curve 26 by adjusting the wobbling intensity, however, since the spurious 24 is also increased to the level of the spurious 23, it is difficult to determine the optimal wobbling intensity.
Viewing a screen display implementing the wobbling from a location farther than the adequate viewing distance corresponds to observing e.g. the image signal indicated by the curve 27 in FIG. 11 with the response indicated by the curve 30 in FIG. 12, so that the image signal 27 has a further reduced high band, resulting in a blurred image.